This invention relates generally to fibrous material pulp processing and more particularly to removal of a pulp sheet from a filter surface.
In processing of fibrous material, washing and/or filtering is typically performed on the pulp in the form of a pulp sheet or aggregate. The pulp sheet is typically formed on a foramenous filter surface in response to a pressure gradient across the filter. Filters may be configured as flat belts, flat or contoured disks, cylindrical drums, or other shapes.
The pressure gradient may be created by a vacuum maintained upon the downstream, inner, or bottom side of the filter, by a slight positive pressure maintained upon the pulp slurry feedstock on the upstream, outer, or top side of the filter, or by a gravity drain. In either case, the aggregate forms as the pulp is filtered from the slurry when the liquor passes through the foramenous filter surface in response to the pressure gradient described.
Following the washing and thickening operations, the pressure differential is interrupted and the pulp sheet is removed from the filter. Removal is typically accomplished with the aid of a doctor device which separates the sheet from the filter surface and directs it to a conveyor mechanism. This doctor device usually consists of a thin blade or a counterrotating toothed roll usually assisted by a water, steam, or air nozzle. Due to dimensional defects, mechanical deflections, and thermal gradients, the filter surface is slightly distorted. If a steel doctor blade or roll is used, contact between it and the filter surface can cause damage to the filter and to the doctor device. If plastic or composite blades or rolls are used and if there is contact, they can wear away and deposit plastic particles or shreds in the pulp sheet. These plastic inclusions are defects in the pulp which cannot be washed out. In order to avoid damage to the pulp, the filter, or the doctor device, a gap is maintained between them.
Formation of the sheet on the filter surface due to the pressure differential results in dimples on the bottom side of the aggregate which are caused by some of the pulp fibers partially entering the perforations on the foramenous filter surface. The inherent stickiness of a wet pulp sheet together with the pulp dimples creates a high degree of adhesion between the pulp sheet and the filter.
In overcoming the adhesive force, the doctor device sometimes splits the pulp sheet and allows some fraction of the adhering pulp to pass beneath the doctor device. The adherent pulp may build up and jam beneath the doctor causing damage to either the doctor, the filter, or both. It may also cause a loss of filtering and/or washing capacity due to plugging of the filter surface perforations. Pulp which passes beneath the doctor must be removed and recycled. This results in a loss of capacity.
The foregoing illustrates limitations known to exist in present methods of pulp sheet removal. Thus, it is apparent that it would be advantageous to provide alternatives directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.